(1) Field of the Invention
The present invention relates to an optical amplifier suitable for controlling the passing-wavelength characteristic of amplified wavelength-division-multiplexed (WDM) light, and relates to a passing-wavelength characteristic control method in that optical amplifier, and an optical transmission system.
(2) Description of the Related Art
With a rapid increase in the number of Internet users and start of distribution service such as images, etc., there is an increasing strong demand for continuous access to high-speed optical networks that are the backbone of Internet. A wavelength-division multiplexing (WDM) system, which multiplexes and transmits a plurality of optical signals of different wavelengths through a single optical fiber (hereinafter referred to as a fiber), has been desired to be put to practical use, because it can increase its transmission capacity in proportion to the number of wavelengths.
Optical networks with multiple fibers connected thereto are provided with WDM-light amplifying equipment (hereinafter referred to as equipment) that transmits high-speed, large-capacity, and long-distance data. This equipment has an optical amplifier. This optical amplifier compensates for a loss of an optical signal, whereby the optical signal is relayed. The main cause of loss lies in the scattering of an optical signal outside a fiber or absorption by impurities within a fiber.
A typical example of the optical amplifier is an erbium-doped fiber amplifier (EDFA). This erbium-doped fiber amplifier is an optical amplifier consisting of an erbium-doped fiber. If light from a pump laser enters through an erbium-doped fiber, the light excites erbium atoms, which then amplify an optical signal as it passes through the erbium-doped fiber.
Conventional optical amplifiers compensate optical signals by the two following methods (J1) and (J2) in order to eliminate a deviation in gain and a deviation in loss for each wavelength:
(J1) Compensation for the undulated gain profile of an erbium-doped fiber, and
(J2) Compensation for a primary tilt due to transmission line loss and stimulated Raman scattering.
The gain profile in (J1) means the gain-wavelength characteristic (where gain depends upon wavelength) of an erbium-doped fiber. The shape of this gain profile is shown, for example, in FIG. 24A and is determined by the excitation ratio of an optical fiber (δ1/δ0 where δ1 represents the ion density in an upper level and δ0 represents the total ion density). To make the gain-wavelength characteristic flat, the number of holes for radiation needs to be constant regardless of input optical power. Also, the operating state of an optical amplifier at this time is equivalent to an automatic gain control (AGC) state that does not depend on input optical power. However, in this AGC state, there are cases where excitation becomes unnecessary (waste).
For instance, in the case where there is sufficient output optical power at 0 (dBm), input light with an input optical power of −10 (dBm) has a value of 10 (dBm) after amplification, if the excitation ratio remains at 20 (dB). Therefore, to make the output wavelength characteristic flat, the power equivalent to 10 (dBm) obtained by amplification needs to be attenuated with an attenuator. In this case, a network administrative operator (hereinafter referred to as an administrative operator) carries out automatic level control and attaches an external optical filter to the output side of an erbium-doped fiber amplifier, thereby making the shape of the gain profile flat. The use of this external optical filer can minimize power consumption. Note that the optical filter will be described later.
If the number of relay stages (amplification stages) is increased, the gain profile is accumulated and unfavorably affects the qualities of optical signals. Because of this, in the case where transmission lines are constructed of erbium-doped fibers, optical amplifiers compensate optical signals by employing band-fixed filters that have the inverse of the gain profile of the erbium-doped fibers.
Stimulated Raman scattering in (J2) is the non-linear phenomenon of transmission fibers in which optical power is shifted from a high frequency side to a low frequency side. When monochromatic light with great power is irradiated to an optical fiber, it interacts with an optical phonon, and consequently, coherent Stokes light with a wavelength shifted by a specific quantity occurs by stimulated emission. Stimulated Raman scattering uses this phenomenon. That is, stimulated Raman scattering amplifies an optical signal by stimulated emission, by setting the wavelength of monochromatic light so that the wavelength of Stokes light becomes the same as that of the optical signal. And with stimulated Raman scattering, compensation for a primary tilt of the gain profile is made. Note that because stimulated Raman scattering occurs in a fiber having a dielectric substance, an administrative operator adjusts the output of excitation light, etc., according to the type of transmission line used.
Also, Raman amplification is amplification utilizing a stimulated Raman dispersion effect and can obtain a desired gain-wavelength characteristic by adjusting excitation light power and oscillation wavelength.
Conventional optical transmission systems connect erbium-doped filters and optical filters in multiple stages, control the excitation of the light input to the erbium-doped filters, and employ a band-fixed filter to suppress noise light that is outside that band.
In this way, conventional optical transmission systems can utilize components that are now in use, and realize reliability and low prices. The optical transmission systems further compensate for a low-noise tilt (which is a parameter representing the primary tilt of the gain of an optical amplifier) by using a Raman fiber amplifier together. Because the output level after amplification is not uniform in wavelength, each distant station monitors the primary tilt of the gain. Unless otherwise noted, distant stations include repeaters, repeater stations, repeater nodes, and optical WDM stations (terminal stations).
Furthermore, there has been proposed a method employing an optical filter. This optical filter is a variable gain equalizer that can variably tune the amount of gain or loss for each wavelength. This optical filter has small power consumption as an optical amplifier and uses active components that can tune gain and loss. This function is realized, for instance, by variable filters (trade name: active gain equalizer), etc. In the method employing the optical filter, the optical amplifier controls the wavelength characteristic of the variable filter by monitoring output optical power of desired wavelengths with an optical spectrum analyzer (e.g., see Japanese Laid-Open Patent Publication No. HEI 10-276173).
In the multiplexing method disclosed in the aforementioned publication, signal light with wavelengths multiplexed within a multiplexer unit is transmitted through a tunable filter, and the level is measured for each wavelength with a photodiode. Based on the detected levels, a setting part adjusts a variable attenuator so that the levels are tuned to a fixed value. Thus, the levels of WDM light can be automatically tuned at all times.
FIG. 24B shows an example of an optical power monitor employing an optical spectrum analyzer. At the gain block 200a of an optical amplifier 200 shown in the figure, an input optical signal is amplified. The amplified light is branched for each wavelength at a demultiplexer 200b. The branched optical signals are compensated at a plurality of optical filters 200c whose pass-bands are different from one another. The compensated optical signals are multiplexed at a multiplexer 200d. The multiplexed signal is again amplified at a gain block 200a and output to an optical amplifier of the next stage (not shown). The WDM light output from the gain block 200a is monitored at an optical spectrum analyzer (OSA) 200e, whereby the quantities of loss of the optical filters 200c are tuned to a previously set compensation quantity.
The method employing the optical filters 200c can reduce the power consumption and cost of redundant components. Therefore, this method can eliminate a deviation in gain for each wavelength that conventional optical amplifiers have.
Conventional optical transmission systems flatten the wavelength characteristics of repeater's amplification and amplified output signals, by a combination of erbium-doped fiber amplifiers and Raman fiber amplifiers. Also, optical signals are compensated by optical filters alone, or by a combination of optical filters and stimulated Raman scattering.
In the case of a combination of optical filters and stimulated Raman scattering, when stimulated Raman scattering is employed as a second-stage amplifier, the wavelength characteristic of the erbium-doped fiber of the first stage has an unfavorable influence on the stimulated Raman scattering of the second stage. More specifically, if the shape of the gain profile of the erbium-doped fiber changes according to the secondary operating point, an administrative operator cannot cause the gain obtained by the stimulated Raman scattering of the second stage to be active or tunable according to wavelengths. This reason is because the gain profile obtained by stimulated Raman scattering is determined by the transmission line type used. Therefore, if stimulated Raman scattering is used as a second-stage amplifier, compensation for only a wavelength band lost is made.
Because of this, an administrative operator adjusts the excitation light output of the erbium-doped fiber amplifier of the first stage so that the secondary operating point does not change. The administrative operator also performs repeater's amplification by using a Raman fiber amplifier together as occasion demands. More specifically, each repeater controls filter characteristics to the optimum state by employing an optical wavelength monitor.
However, in the case of using optical wavelength monitors, optical transmission systems have to transfer monitoring light with a specific wavelength at all the bands of an optical amplifier. Besides, the monitoring light is unnecessary because it is used solely for the purpose of monitoring the gain-wavelength characteristic of the optical amplifier. Moreover, the administrative operator must cause customers to purchase redundant equipment such as an optical spectrum analyzer for monitoring, etc. Therefore, the initial introduction cost that is imposed on customers is increased.
Also, in the filter control method that employs an optical spectrum analyzer, etc., optical amplifiers feed monitored results back to the upstream side of a transmission line. This feedback control is accurate, but is susceptible to external disturbance when increasing or decreasing wavelength channels (channels allocated to light wavelengths). This filter control method requires a considerable time until control becomes stable. In general, if interrupting transfer is needed when networks are connected in mesh form, fault recovery operation is indispensable and that interruption transfer is also time-consuming. Therefore, gain tuning that employs optical wavelength monitors is temporally difficult and is not practical control.
Furthermore, the above-described optical amplifiers, such as an erbium-doped fiber amplifier, Raman fiber amplifier, etc., are not efficient in operation and contain many redundant portions that increase both power consumption and equipment costs. Erbium-doped fiber amplifiers, and excitation lasers for stimulated Raman scattering, consume great power. Therefore, each device requires a means of radiating heat generated by great power. This heat-radiating means increases the size and cost of the transmission system. Particularly, the large size of Raman fiber amplifiers causes increased costs.